Carbohydrates

Question 1

What are examples of monosaccharides?

Answer 1

Glucose, galactose and fructose

Question 2

What are examples of polysaccharides?

Answer 2

Maltose, sucrose abd lactose

Question 3

Where are the 3 areas that digestion occurs?

Answer 3

Mouth, duodenum and jejunum

Question 4

Does digestion occur in the stomach?

Answer 4

Nope

Question 5

How are carbohydrates digested in the mouth?

Answer 5

Salivary amylase enzyme hydrolyses alpha (1->4) bonds of starch

Question 6

How are carbohydrates digested in the duodenum?

Answer 6

Pancreatic amylase enzyme hydrolyses alpha (1->4) bonds of starch

Question 7

How are carbohydrates digested in the jejunum?

Answer 7

Enzymes hydrolyse alpha (1->6) bonds, which removes glucose and hydrolyses sucrose and lactose

Question 8

What are the 3 products of digestion?

Answer 8

Glucose, galactose and fructose

Question 9

How is glucose absorbed?

Answer 9

Glucose is absorbed when it travels between the intestinal lumen into blood via epithelial cells. It is driven through the lumen by a high EC Na+ concentration which is maintained by an ATP-driven Na+ pump

Question 10

How is galactose absorbed?

Answer 10

Though concentration gradients

Question 11

How is fructose absorbed?

Answer 11

Its absorbed through the lumen into blood by binding to the channel protein GLUT5, where it moves down its concentration gradients

Question 12

What is an isoenzyme?

Answer 12

An enzyme that catalyses the same reaction but has different Km’s and Vmax’s

Question 13

What is the connection between hexokinase and glucokinase

Answer 13

They are isoenzymes

Question 14

What are the differences between hexokinase and glucokinase?

Answer 14

Hexokinase- low Km and Vmax, can bind to glucose when [glucose] is low
Glucokinase- high Km and Vmax, can only bind to glucose when [glucose] is high

Question 15

When does glucokinase bind to glucose?

Answer 15

Quickly after a meal and traps as much as possible in the liver

Question 16

Where is glucokinase found?

Answer 16

In the liver

Question 17

What is glycogen?

Answer 17

It is a polymer of glucose linked by alpha(1->4) subunits with alpha(1->6) branches every 8 to 12 residues. It is mostly found in liver and skeletal muscle

Question 18

How is glycogen produced?

Answer 18

1. Glycogenin covalently binds glucose from UDP-glucose to form chains of approx. 8 sub-units
2. Glycogen synthase extends the chains
3. Glycogen-branching enzymes break the chain down to form 1-6 branching points

Question 19

How is glycogen degraded?

Answer 19

Glycogen phosphorylase moves glucose subunits one at time as G-6-P from the non-reducing ends of the branches

Question 20

What happens to glycogen in skeletal muscle?

Answer 20

It is not converted into glucose in the blood as there is no G-6-phosphatse so it enters glycolysis

Question 21

What happens to glycogen in the liver?

Answer 21

It is converted into G-6-P then glucose in the blood as a result of a drop in [BG]

Question 22

What is the function of glycolysis?

Answer 22

It is a metabolic pathway that saves some Ep from glucose as ATP via substrate level phosphorylation

Question 23

What is the net gain in glycolysis?

Answer 23

2x ATP, 2xNADH and 2x pyruvate molecules

Question 24

What happens to the pyruvate molecules produces in glycolysis?

Answer 24

It depends on what the body needs

Question 25

What 2 substances are involved in dehydrogenase catalysed reactions?

Answer 25

Lactate (lactate -> pyruvate) and pyruvate (pyruvate -> acetyl CoA)

Question 26

How is lactate converted into pyruvate?

Answer 26

By NAD+ during gluconeogenesis

Question 27

When is blood lactate used up?

Answer 27

During exercise

Question 28

What is the fate of blood lactate?

Answer 28

Gluconeogenesis converts lactate -> glucose in the liver due to a lack of supply of oxygen for oxidative phosphorylation to produce ATP so ATP is produced via substrate level phosphorylation which also produces lactate

Question 29

What is the Cori cycle?

Answer 29

Cycle between glycolysis in the tissues and gluconeogenesis in the liver. It is a metabolic pathway in which lactate produced by anaerobic glycolysis in the muscles moves to the liver and is converted to glucose

Question 30

What is the fate of absorbed galactose?

Answer 30

It joins glycolysis after through the conversion of G-1-P by UDP

Question 31

What is the fate of absorbed fructose?

Answer 31

It joins glycolysis by using ATP to form F-1-P then glyceraldehyde which then uses another ATP molecule to form glyceraldehyde-3-P

Question 32

What are the precursors of gluconeogenesis?

Answer 32

Lactate, amino acids and glycerol

Question 33

What are the functions of gluconeogenesis?

Answer 33

Converts lactate into glucose in the liver

Question 34

Is gluconeogenesis an exact reversal of glycolysis?

Answer 34

Nope, as 3/10 of the reactions in glycolysis are not reversible (not feasible) as they have a -ve delta G

Question 35

What is the process of gluconeogenesis?

Answer 35

The 3 reactions that are't feasible are bypassed by enzymes that catalyse 4 different irreversible reactions

Question 36

What is the pentose phosphate pathway?

Answer 36

A metabolic pathway that produces NADPH and pentose sugars which are needed for nucleic acid synthesis and for the metabolism of the small amount of pentose sugars in the diet

Question 37

What are the 2 phases of the pentose phosphate pathway?

Answer 37

Oxidative phase and a non-oxidative phase

Question 38

What is the oxidative phase of the pentose phosphate pathway?

Answer 38

It is irreversible, generates NADPH and converts G-6-P -> pentose phosphate

Question 39

What is the non-oxidative phase of the pentose phosphate pathway?

Answer 39

It is reversible, interconverts G-6-P and pentose phosphate and forms many carbon sugars